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How to harden a Memcached container (and is memcached:1.6-alpine safe for untrusted workloads?)

A cache sits in the hot path of every request, which is exactly why its container should be tight. A stock docker run memcached:1.6-alpine starts ahead of most images (it runs non-root out of the box) but is still not a boundary to trust around an untrusted network. Graded on IronClaw's seven-dimension containment scale, the default configuration scores 63 of 100, grade C (partial). Higher is safer. A couple of runtime flags take the same image to 89 of 100, grade B, one point off an A, and the one dimension it cannot reach is the one a cache needs by definition (clients must connect to it). Here are the exact gaps and fixes from the scan data.

Every number here comes from a read-only docker inspect of memcached:1.6-alpine, the same data behind its isolation scorecard. No workload is executed. How scoring works →

Where the default configuration leaks

ironctl scan grades seven independent containment boundaries. On a default docker run memcached:1.6-alpine, three fail or warn:

Dimension Verdict Score What the scan found
Non-root user (uid != 0) ✅ PASS 15/15 runs as memcache (uid != 0)
Dropped capabilities ❌ FAIL 4/20 default capability set retained (CAP_NET_RAW, CAP_MKNOD, and more)
Seccomp profile ✅ PASS 15/15 seccomp profile active
Network isolation / egress ⚠️ WARN 4/15 network=bridge: outbound egress is possible
Read-only root filesystem ❌ FAIL 0/10 root filesystem is writable
No docker.sock exposure ✅ PASS 15/15 no control socket mounted
No shared host namespaces ✅ PASS 10/10 no host PID/IPC/network sharing

Memcached already runs as the non-root memcache user, which is why it clears the 48/100 that root-by-default images sit at. The remaining leaks are the retained capabilities and the writable root filesystem. Memcached is purely in-memory, it has no data directory to persist, so there is no reason its root filesystem should be writable at all: a read-only rootfs removes the persistence surface for free, and dropping the default capabilities takes CAP_NET_RAW and friends away from any code that lands via a protocol-parsing CVE.

Harden it: the exact --fix remediation

ironctl scan my-memcached --fix prints one remediation per failed dimension, then one hardened run. For memcached:1.6-alpine:

  • --cap-drop=ALL (Dropped capabilities, +16): drop every Linux capability; add back only what the workload provably needs. Memcached needs none of the defaults.
  • --read-only --tmpfs /tmp (Read-only rootfs, +10): make the root filesystem read-only. Memcached keeps everything in memory, so no writable volume is needed at all, a rare clean win.
  • --user 65532:65532 (Non-root user, already passing at +0): non-root already passes as the memcache user. Pinning an explicit fixed uid is the auditable choice; it does not change the score.
  • Scoped network (Network isolation): --network=none scores the full 15 but is wrong for a cache, clients must be able to connect to it. Any named or bridge network scores 4 of 15 (a WARN, not a fail): a connection path exists. This is the one dimension a cache cannot max out. Contain it anyway: attach a user-defined network scoped to just its clients, with no default route out, so a compromised cache cannot call arbitrary internet addresses.

Before and after

# Before: 63/100, grade C
docker run -d --name memcached memcached:1.6-alpine

# After: 89/100, grade B (scoped private network for its clients)
docker run -d --name memcached-hardened \
  --user 65532:65532 \
  --cap-drop=ALL \
  --security-opt=no-new-privileges \
  --read-only --tmpfs /tmp \
  --network=cache-internal \
  memcached:1.6-alpine

Rescan: ironctl scan memcached-hardened reports 89/100 grade B. A 26-point swing with no custom image build and no volume, just the right flags. The only dimension still short of full marks is the network (4 of 15), because a cache exists to be connected to; network=none would score the last points but leave nothing able to reach it. That is the honest ceiling for a cache, and it is a clean grade B up from the default C.

Verify it on your own cache

# install (Homebrew)
brew install ironsecco/ironclaw/ironclaw

# grade your running container, then print the fixes
ironctl scan my-memcached
ironctl scan my-memcached --fix

ironctl scan also reads a docker-compose.yml service or a Kubernetes manifest, so you can grade the Memcached in your stack, not just a bare docker run.

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